(CNSNews.com) – Both Homeland Security Secretary Secretary Jeh Johnson and Centers for Disease Control and Prevention Director Tom Frieden said on Wednesday that 150 people a day arrive in the United States from Liberia, Sierra Leone or Guinea, the three West African countries that have been hit by the Ebola virus.

“When somebody travels from one of those three West African countries, even through a transit point (in Europe), we know where they’re coming from. So we’re able to track this. And we know that on average it’s about 150 passengers a day,” Johnson told CNN’s Wolf Blitzer.

“The number of travelers is relatively small. We’re talking about 150 per day,” Frieden told a news conference on Wednesday. He announced that questionnaires and temperature checks will begin at five major U.S. airports that handle “95 percent of all the 150 travelers per day who arrive from these three countries.”

One hundred-fifty passengers a day from West Africa works out to more than a thousand a week (1,050), 4,500 a month, and 54,750 a year.

Given the 21-day incubation period for Ebola and the 150 people coming in each day, that means that at any given time there could be 21 x 150 people in the U.S. who could be asymptomatically incubating Ebola, and who are free to wander around the country.

That is 3,150 people who at any moment could become symptomatic anywhere in the country and start exposing people.

All passengers flying out of West African countries already undergo temperatures checks and answer questions about their contact with Ebola patients, but the screening doesn’t always work, as the case of Thomas Eric Duncan proves.

Duncan died of Ebola on Wednesday in Dallas, after coming to the U.S. from Liberia on Sept. 20. He reportedly failed to tell airport officials in Liberia about his contact with an Ebola patient, and he did not have a fever when he left that country.

‘Think Ebola’

At Wednesday’s news conference, the CDC’s Frieden told reporters that he expects some people traveling to the U.S. from West Africa to have fevers, but it may not be Ebola:

“In fact, we know that over the past couple of months, about one out of every 500 travelers boarding a plane in West Africa has had a fever. Most of those had malaria. None of those, as far as we know, have been diagnosed with Ebola. So we expect to see some patients with fever, and that will cause some obvious and understandable concern at the airports.”

Frieden said malaria is spread by mosquitoes; it is very common in West Africa; and it is characterized by a fever that “comes and goes.” “So it would not be surprising if we saw individuals with malaria have a fever after coming back here,” he said.

Even if it turns out to be malaria, Frieden said he wants all U.S. medical practitioners to “think Ebola.”

“We’re working hard to promote now is ensuring that doctors and nurses, pharmacists, health care workers throughout the health care system think Ebola in anyone who has fever and ask whether they have been in west Africa in the past 21 days.”

The Obama administration has ruled out a travel ban or even a quarantine on travelers arriving from West Africa: “We need airlines to continue to operate in West Africa; and we need borders to remain open,” Secretary of State John Kerry said on Wednesday.

Hours later, Frieden stressed that “protecting Americans is our number one priority.”

He also explained the dire situation in Liberia, Sierra Leone and Guinea:

“Liberia has had the most extensive epidemic so far. There have been in some areas of Liberia some decreases in recent weeks, but we don’t know whether those will hold. In Sierra Leone, we’re continuing to see increases in cases that are very concerning. Guinea has seen increase and decreases, and we’re monitoring that very closely.”

Frieden said the challenge for the international community is “how rapidly the disease is spreading.”

He also noted “signs of progress” in West Africa: “For example, we’re seeing more safe burials in Liberia…We’re increasing isolation and treatment capacity. So I think we’re beginning to see that kind of surged response have an impact on the front lines, but it’s going to be a long, hard fight and in West Africa we’re far from being out of the woods.”